1. Field of the Invention
The present invention relates to a zoom lens system consisting of two lens units and which is to be used to pick up an image, for example a zoom lens consisting of two lens units which is to be used in lens shutter cameras.
2. Description of the Prior Art
In recent years, zoom lens systems have been generally used in lens shutter cameras and there has been demand in particular for inexpensive compact cameras. Speaking concretely, optical designs are carried out to permit one to compose lens systems of fewer lens elements and using inexpensive materials. To reduce thicknesses of camera bodies a main stream of lens systems are formed by the so-called collapsible mount type lens systems which are accommodated in camera bodies where airspaces between adjacent lens units, which are moved to change a magnification of the lens system, are narrowed. In order to further reduce thicknesses of camera bodies, it is necessary to shorten the total length of each lens unit, which are moved on an optical axis to change the magnification of the lens system.
There have been proposed a lot of such zoom lens systems, each having a simple composition consisting of two positive and negative lens units. Such zoom lens systems have been disclosed, for example, by Japanese Patents Kokai Publication No. Hei 6-347696, No. Hei 8-248313, No. Hei 990220, No. Hei 9-96761, No. Hei 9-166749, No. Hei 10-104516 and No. Hei 10-104517.
The zoom lens systems disclosed by these patents use aspherical lens elements to reduce the numbers of lens elements to be used, thereby making the lens systems compact. Furthermore, the aspherical lens elements are made of plastic materials to lower manufacturing costs of the zoom lens systems.
However, the zoom lens systems disclosed by Japanese Patents Kokai Publication No. Hei 6-347696, No. Hei 8-248313, No. Hei 9-00220, No. Hei 10-104516 and No. Hei 10-104517 do not correct chromatic aberration sufficiently, thereby allowing chromatic aberration to be varied remarkably by changing magnifications, and are incapable of exhibiting favorable optical performance over entire zooming ranges.
Furthermore, the zoom lens systems disclosed by Japanese Patents Kokai Publication No. Hei 9-90220 and No. Hei 9-96761 have field angles on the order of 56.degree., which are not sufficiently wide at wide positions even though these zoom lens systems have vari-focal ratios of 2 or higher. Moreover, the zoom lens system disclosed by Japanese Patent Kokai Publication No. Hei-96761 does not correct coma sufficiently at the wide position, thereby being incapable of exhibiting favorable optical performance.
Each of the zoom lens systems disclosed by the prior art uses a plastic lens element having a weak refractive power in a positive lens unit, but adopts a plastic lens element having a certain degree of refractive power in a negative lens unit which has refractive index and surface shape changes due to temperature and humidity variations, thereby posing a problem to change a refractive power of the lens system, and aberrations in the lens system.
Furthermore, vari-focal ratios exceeding 2 are desired for zoom lens systems for lens shutter cameras. Since it is desired that a photographic lens system be compact and light in weight even when it has a high vari-focal ratio, it is attempted to shorten the diameter of the lens system and narrow the airspace between lens units adjacent to each other so that the total length of the lens system is accommodated in the camera body in the so-called collapsed condition. Shortening the total length of the lens system in the collapsed condition constitutes a theme important for development of a lens system. For compact configuration of a zoom lens system, it is general to use an aspherical lens element to reduce the number of lens elements to be used in the lens system.
A zoom lens system disclosed by Japanese Patent Kokai Publication No. Hei 8-304701, for example, is known as a zoom lens system which uses an aspherical lens element to reduce the number of lens elements used therein, and has a zooming vari-focal ratio of 2.5 or higher.
Furthermore, it is practised to use plastic materials for aspherical lens elements so that the aspherical elements can be manufactured at low costs and have light weights. Zoom lens systems disclosed by Japanese Patents Kokai Publication No. Hei 6-130298, No. 7-261078, No. Hei 9-152549 and No. Hei 9-5627, for example, are known as zoom lens systems which are configured compactly and are light in weight; using aspherical lens elements, and having vari-focal ratios of 2 or higher.
When an aspherical lens element is made of a plastic material, however, there is posed a problem that the shape and the refractive index of the plastic lens element are changed due to temperature and humidity variations. Speaking more concretely, the focal length of the plastic lens element is changed due to environmental variations, thereby distorting a focus surface of a lens system. This problem is more noticeable with a plastic lens element that has a stronger refractive power; and plastic lens elements are used in a larger number.
The zoom lens systems disclosed by Japanese Patent Kokai Publication No. Hei 6-130298 and No. Hei 9-5627 are configured to reduce influences on the plastic lens elements due to temperature and humidity variations by imparting weak refractive powers to the plastic lens elements, but allow focus surfaces to be remarkably varied at telepositions, and cannot completely prevent the influences due to the temperature and humidity variations. Furthermore, each of the zoom lens systems uses a plurality of plastic lens elements, and is liable to be influenced by the temperature and humidity variations.
The zoom lens system disclosed by Japanese Patent Kokai Publication No. Hei 7-261078 uses a plastic lens element that has a strong refractive power, thereby being liable to be influenced due to temperature and humidity variations. Furthermore, lens units which are moved to change a magnification of the lens system have relatively long total lengths on an optical axis, thereby enlarging the thickness of the camera body in a collapsed condition of the zoom lens system.
The zoom lens system disclosed by Japanese Patent Kokai Publication No. Hei 9-152549 adopts a plastic lens element which has a weak refractive power in order to reduce the influences on the plastic lens element due to the temperature and humidity variations, but allows the focus surface to be remarkably varied at the teleposition, and cannot completely prevent the influences due to the temperature and humidity variations. Furthermore, lens units which are moved to change a magnification of the lens system have a relatively long total length on an optical axis, thereby enlarging the thickness of the camera body in an collapsed condition of the zoom lens system.
On the other hand, the zoom lens system disclosed by Japanese Patent Kokai Publication No. Hei 8-304701, which is to have the zooming vari-focal ratio exceeding 2.5, uses lens units composed of a large number of lens elements, and has a large total length on an optical axis, thereby enlarging the thickness of the camera body in a collapsed condition of the lens system.